1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus, and more particularly to an image forming apparatus including a conductive brush member that cleans a surface of a charging member, such as a charging roller, which uniformly charges a surface of a latent image carrier.
2. Discussion of the Related Art
In an image forming apparatus that has been generally used, an electrostatic latent image formed on a latent image carrier is developed as a toner image by a developing device. Subsequently, the toner image is transferred and fixed onto a recording sheet. In this image forming apparatus, a cleaning device including, for example, a cleaning blade, removes residual toner, which has not been transferred from the latent image carrier to the recording sheet, from the latent image carrier. Thereby, the surface of the latent image carrier is prepared for a next image formation.
A surface of a photoreceptor functioning as a latent image carrier is uniformly charged by a charging device. Then, an image writing device irradiates the surface of the photoreceptor with a light and forms an electrostatic latent image on the surface of the photoreceptor. Generally, a charging device that charges the surface of the photoreceptor includes a charging member, such as a charging roller. The charging roller is in contact with or adjacent to the surface of the photoreceptor, and a voltage is applied to a position between the charging roller and the photoreceptor. The charging roller of this type is practically used in view of reduction of ozone and electric power. In this charging device, when foreign substances, such as toner and paper powder, are adhered onto the surface of the charging roller, the charging roller may not uniformly charge the surface of the photoreceptor. Recently, with an increasing demand for enhancing an image quality and resolution, a toner having a small particle diameter has been often used in a development process. However, the toner having a small particle diameter typically causes a cleaning failure in which a cleaning device may not adequately remove the toner from the surface of the photoreceptor. In this condition, residual toner remaining on the surface of the photoreceptor adheres to the above-described charging roller, so that the charging roller may not uniformly charge the surface of the photoreceptor. To address this problem, Published Japanese patent application No. 2002-221883 describes a cleaning device that removes foreign substances from a surface of a charging roller by use of a brush roller.
As a developing device that develops an electrostatic latent image formed on a photoreceptor with a developer, a so-called two-component type developing device is generally used. In the two-component type developing device, a developer carrier carries a two-component developer (hereafter referred to as a “developer”) including toner and magnetic carrier thereon. An electrostatic latent image formed on a photoreceptor is developed by forming a magnetic brush including magnetic carrier holding toner on the surface of the developer carrier by the action of a fixed magnetic pole in the developer carrier.
Further, in the two-component type developing device, a developer regulating member, such as a doctor blade, regulates a layer thickness of the developer carried on the developer carrier. The developer having a predetermined layer thickness is conveyed to a developing region where the developer carrier faces the photoreceptor by the movement of the surface of the developer carrier. At this time, the magnetic carrier and toner may scatter by the influence of a centrifugal force exerted on the developer and an airflow in the developing device (hereafter referred to as a “developer scatter”). Especially, if carrier and toner having small particle diameters are used, a developer scatter tends to occur. To prevent the developer scatter, a developer scatter preventing member is provided to cover a developer layer that has passed the developer regulating position where a developer regulating member regulates the layer thickness of the developer carried on the developer carrier. For example, Published Japanese patent application Nos. 2002-278287 and 2002-287503 describe a developing device in which a developer scatter preventing member is provided.
FIG. 1 is a schematic view of a background developer scatter preventing member 110a. Referring to FIG. 1, one end of a developer scatter preventing member 110a is fixed onto an edge portion of a casing (not shown) to cover a developer layer D which is deposited on a developing roller 141 functioning as a developer carrier and which has passed a position where a developer regulating member (not shown), such as a doctor blade, regulates a layer thickness of the developer on the developing roller 141. The casing has an opening exposing a portion of the developing roller 141 and has the edge portion adjoining the opening. Further, the developer scatter preventing member 110a is flexed such that another end of the developer scatter preventing member 110a contacts a surface of a photoreceptor 101 to block a gap between the edge portion of the casing and the surface of the photoreceptor 101. With this configuration, the developer scatter preventing member 110a can prevent the developer from scattering at the position on an upstream side of a developing region where the developing roller 141 faces the photoreceptor 101, in a direction of conveying the developer (hereafter referred to as a “developer conveying direction”).
For example, Published Japanese patent application No. 10-268639 describes an image forming apparatus including an elastic sheet like the above-described developer scatter preventing member 110a and elastic seal members to block a gap between a photoconductive drum and a developer carrier. The elastic seal members press-contact non-image formation areas on respective outer circumferential surfaces of the photoconductive drum and the developer carrier, which are respectively located on both end portions of the photoconductive drum and the developer carrier in each of rotation shaft directions of the photoconductive drum and the developer carrier. With this configuration, the elastic sheet prevents a developer from scattering at the position on an upstream side of a developing region in a developer conveying direction, and the elastic seal members prevent the developer from scattering from the both end portions of the developer carrier.
The developer scatter preventing member 110a can prevent the developer from scattering from the developer layer (D) on the developing roller 141 in an early period. However, as the number of image formations increases, toner (T) adheres to a surface (hereafter referred to as a “rear surface”) of the developer scatter preventing member 110a facing the developer layer (D), so that the toner (T) accumulates on the rear surface of the developer scatter preventing member 110a (hereafter referred to as “accumulation of toner”). The accumulation of toner, that is, agglomeration of toner, falls to the developing region immediately after the start of rotation of the developing roller 141 and when an impulse is given to the agglomeration of toner in an image formation process. If the agglomeration of toner adheres to a non-image area and an image area on the photoreceptor 101, an output image is stained. Further, a partial omission of an output image may occur due to a poor transfer efficiency of the agglomeration of toner and disturbance of a transfer electric field around the agglomeration of toner. Moreover, if toner, which has passed through the developing region, accumulates on a sheet conveying guide, a transfer sheet may be stained. Further, if toner accumulates on the rear surface of the developer scatter preventing member 110a, the position of the developer scatter preventing member 110a may shift due to the weight of the agglomeration of toner. Thereby, a contact pressure of the developer scatter preventing member 110a against the surface of the photoreceptor 101 changes. The developer may consequently leak out from the portion of the developer scatter preventing member 110a which contacts the surface of the photoreceptor 101 with low pressure. The leaked developer may scatter from the developing device.
If a magnetic brush including magnetic carrier holding toner rises by the action of a magnetic pole in the developing roller 141 at the most downstream position of the developer scatter preventing member 110a in the developer conveying direction, the risen magnetic brush pushes the most downstream portion (i.e., the leading edge portion) of the developer scatter preventing member 110a. If the developer scatter preventing member 110a is located above the developing region as shown in FIG. 1, the leading edge portion of the developer scatter preventing member 110a is pressed upward by the pushing force of the risen magnetic brush. In this condition, the friction between the pushed-up leading edge portion of the developer scatter preventing member 110a and the surface of the photoreceptor 101 may cause damage to the surface of the photoreceptor 101, an abnormal image such as a black streak image, and a cleaning failure. Above all, the edge portion of the developer scatter preventing member 110a is significantly pushed up by the risen magnetic brush. Therefore, a gap is formed between the edge portion of the developer scatter preventing member 110a and the surface of the photoreceptor 101. The developer may scatter from the developing device through the gap formed between the edge portion of the developer scatter preventing member 110a and the surface of the photoreceptor 101.
Further, the present inventor found that an image may be deteriorated when a brush roller is used as a cleaning device that cleans a surface of the above-described harging member. The cause of the deterioration of an image is considered as follows.
When removing foreign substances adhered onto a surface of a charging member by a brush roller, the cleaning ability of the brush roller is enhanced by use of an electrostatic force. Most of the foreign substances adhered onto the surface of the charging member are charged with an opposite polarity to that of a charging bias applied to the charging member. For these reasons, a conductive brush roller is often used as a cleaning device. The potential of the conductive brush roller may have a polarity equal to that of a charging bias applied to the charging member, and thereby the conductive brush roller may mechanically and electrostatically remove the foreign substances, which are charged with an opposite polarity to that of the potential of the conductive brush roller, from the surface of the charging member. When using a brush roller for a long period of time, a brush of the brush roller may fall from a core metal portion of the brush roller, and the fallen brush may be adhered onto a surface of a photoreceptor via the charging roller. In this condition, the fallen brush may be conveyed to a position where a developer scatter preventing member contacts the surface of the photoreceptor by the movement of the surface of the photoreceptor, and may stay at the position with the fallen brush sandwiched between the developer scatter preventing member and the surface of the photoreceptor. The brush, which is sandwiched between the developer scatter preventing member and the surface of the photoreceptor, contacts a magnetic brush in a developing region, and the charge on the photoreceptor is leaked to the magnetic brush via the fallen brush. As a result, an electrostatic latent image formed on the surface of the photoreceptor may be distorted, resulting in a deterioration of image quality.
In order to prevent the developer scatter by the developer scatter preventing member, it is preferable that the surface of the leading edge portion of the developer scatter preventing member is brought into intimate contact with the surface of the photoreceptor. However, in this condition, the above-described fallen brush may not pass through the developer scatter preventing member and tends to be sandwiched between the surface of the photoreceptor and the developer scatter preventing member. As a result, an electrostatic latent image formed on the surface of the photoreceptor may be disturbed.
Therefore, the present inventor determined it is desirable to provide an image forming apparatus in which a high quality image can be formed by preventing an image deterioration caused by a fallen conductive brush and by controlling a developer scatter over a long time period.